The UCMJ is tailored to, in many ways, allow commanders at all echelons to effect a command climate as they see fit. Some commanders at higher echelons will reserve adjudication of certain offenses to themselves, and during my years I saw adultery treated in very different ways. I actually knew of one commander who imposed an Article 15 with the absolute minimum punishment (suspended at that), remarking that it was an offense and the evidence led him to believe that it had been commited, and as such he was duty bound to impose punishment, but other than that he saw the offense as silly. I knew other commanders who viewed it as an egregious offense....
However, in a very quiet move, in 2002, President Bush adopted many of the changes that were proposed by President Clinton. In addition to the Elements of Proof," the "Explaination" section under this offense now requires commanders to consider the following factors when determining whether or not the offense of "adultery" constitutes a crime:
The accused's marital status, military rank, grade, or position;
The co-actor's marital status, military rank, grade, and position, or relationship to the armed forces;
The military status of the accused's spouse or the spouse of co-actor, or their relationship to the armed forces;
The impact, if any, of the adulterous relationship on the ability of the accused, the co-actor, or the spouse of either to perform their duties in support of the armed forces;
The misuse, if any, of government time and resources to facilitate the commission of the conduct;
Whether the conduct persisted despite counseling or orders to desist; the flagrancy of the conduct, such as whether any notoriety ensued; and whether the adulterous act was accompanied by other violations of the UCMJ;
Whether the accused or co-actor was legally separated; and
Whether the adulterous misconduct involves an ongoing or recent relationship or is remote in time.
The negative impact of the conduct on the units or organizations of the accused, the co-actor or the spouse of either of them, such as a detrimental effect on unit or organization morale, teamwork, and efficiency;
What this means is that many incidents of "adultery" may not be considered a punishable "crime" in the military, unless there is some kind of direct negative impact on the military itself.
This does not mean, however, that military members are free to shack up with whomever they please. Commanders have a lot of discretion when it comes to administrative procedures, and administrative actions (such as reprimands, denial of promotions, performance report remarks, etc.) are not governed by the relatively strict legal requirements of the UCMJ or MCM.